The present invention relates to methods and systems for locating the source of a radio frequency (RF) transmission, and more particularly to a method and system for accurately locating a mobile cellular radio through the novel use of a grid of radio receivers and the Global Positioning System (GPS).
Cellular mobile radio systems are well known. Such systems use a plurality of base stations for broadcasting paging signals to and for maintaining voice communications with mobile radio equipped units. Such mobile radios may include automobile carried cellular mobile telephones, portable cellular telephones, pagers, dispatch and/or data link radios, and a variety of other RF broadcasting equipment that may be mobile and for which a position may be sought. Most such radios are individually identified by a code or call number. Once turned on, each radio transmits an identifying signal that includes the code so that the radio may be contacted if a call is directed to it. That is, the user of the radio does not have to be actively engaged in a conversation on the radio for a signal from the radio to be received by the base stations.
Frequently, it is desirable that the position of the mobile radio be determined quickly and without the knowledge of the mobile radio's possessor. For example, police may benefit from the knowledge of the position of a mobile radio that may be carried by a suspect (e.g., drug dealer) or by knowing the position of a stolen automobile carrying a mobile radio.
Various systems for determining the position of mobile radios exist in the prior art. However, such systems are not accurate enough for the needs of all potential users. For example, in law enforcement applications it is desirable that the location of the mobile radio be known within 100 meters (less than a city block) so that police may be dispatched efficiently and so that the movement of the radio may be tracked on an appropriate map display.
The accuracy of prior art systems is degraded for various reasons, including the fact that many cellular radio signals are of very short duration, sometimes a second or less. Prior art systems require more time than this to integrate the signal so that an accurate location may be determined.
This problem is exacerbated in the presence of multipath interference for which prior art systems need even longer integration times. Prior art systems use techniques for obviating multipath in which multiple samples are integrated over time, assuming the specular receipts will not be stationary and will have a zero mean over the sample size. As is known, this assumption may be false, depending on geographical conditions and integration time.
A further disadvantage of the prior art is that many of the systems are not passive. That is, some action by the user of the mobile radio may be needed to initiate or maintain tracking by a base unit. For example, the user may have to initiate a special signal that may be detected and tracked. It is desirable that locating systems available to law enforcement not rely on action by a mobile radio user.
Accordingly, it is an object of the present invention to provide a novel method and system for determining the position of a mobile radio that obviates the problems of the prior art.
It is a further object of the present invention to provide a novel method and system for accurately locating a mobile cellular radio that uses an accurate time source to determine the actual time of arrival of a radio signal and to accurately determine the position of a grid of radio receivers.
It is another object of the present invention to provide a novel method and system for accurately locating a mobile cellular radio in which the received radio signals are stripped of interfering multipath and cochannel signals so as to provide a more accurate time of arrival of the received signal.
It is yet another object of the present invention to provide a novel method and system of accurately locating a mobile cellular radio that uses the Global Positioning System to provide an accurate time of receipt of a radio signal from a mobile radio from which the multipath signals have been removed.
It is still another object of the present invention to provide a novel method and system of accurately locating a mobile cellular radio in which a positional accuracy of one hundred meters or less may be achieved with signals having a duration of a second or less.
It is yet a further object of the present invention to provide a novel method and system of accurately locating a mobile cellular radio in which deleterious multipath effects are mitigated without long integration times and without assuming that multipath has a zero mean over the sample size.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments. dr